Debates are often characterised by the extent to which they generate light or heat.

But the heat is all about light in the great dress debate. After two Scottish friends sought the wisdom of Twitter and Facebook in a disagreement the colour of a wedding frock a kind of collective obsession swept the internet over the colour of a dress.

This is only partly a question of the way the dress is lit. The fascination lies in the fact that friends, relatives, workmates, and of course the inevitably celebrities apparently see something different when they look at the same picture.

In one respect this is trivial, of course, compared with the other issues of the day. Whether the dress is blue and black or gold and white scarcely matters. But it is remarkable for the unsettling questions it raises. If you see a simple photograph of a dress quite differently from the person sitting next to you, it makes you wonder what other faulty assumptions you are making.

On one level this is a question of how light falls on cones and rods, and how the individual brain processes that, but thrillingly, this debate shows our neighbours may be more different from us than we realise.

It also satisfies a need for shared conversations, which seem to be ever harder to find in our increasingly fragmented cultural landscape.